Common name:  FENBENDAZOLE

Type: veterinary medecine
Chemical class: benzimidazole

CHEMICAL STRUCTURE

Molecular structure of FENBENDAZOLE 

 


EFFICACY AGAINST PARASITES

Type of action: broad-spectrum nematicide and taenicide anthelmintic, endoparasiticide
Main veterinary parasites controlled: gastrointestinal and respiratory roundworms (= nematodes) and tapeworms

Efficacy against a specific parasite depends on the delivery form and on the dose administered.

Click here for general information on features and characteristics of PARASITICIDES.


DOSING

Click here to view the article in this site with the most common dosing recommendations for fenbendazole used in domestic animals.

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SAFETY

Oral LD50, rat, acute*: >10000 mg/kg
Dermal LD50, rat, acute*: >2000 mg/kg
* These values refer to the active ingredient. Toxicity has to be determined for each formulation as well. Formulations are usually significantly less toxic than the active ingredients.

MRL (maximum residue limit) established for either beef, mutton pork or chicken meat*:

  • CODEX: Yes
  • EU: Yes
  • USA: Yes
  • AUS: Yes

* This information is an indicator of the acceptance of an active ingredient by the most influential regulatory bodies for use on livestock.

Withholding periods for meat, milk, eggs, etc. depend on delivery form, dose and national regulations. Check the product label in your country.

Learn more about fenbendazole safety (poisoning, intoxication, overdose, antidote, symptoms, etc.).

General safety information for antiparasitics is available in specific articles in this site (click to visit):

WARNING

Never use products for livestock on dogs and cats unless they are explicitly approved for both livestock and pets. Pets may not tolerate livestock formulations.

It is obvious that veterinary products are not intended for and should never be used on humans!!!


MARKETING & USAGE

Decade of introduction: 1970
Introduced by: HOECHST (→ INTERVET → MSD Animal Health)
Some original brands: PANACUR
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: Yes, massive
Use in HORSES: Yes, a lot
Use in DOGS and CATS: Yes, moderate to scarce

Main delivery forms

Use in human medicine: No
Use in public/domestic hygiene: No
Use in agriculture: No
Generics available:  Yes, numberless for livestock, a few for pets

SELECTION OF COMMERCIAL BRANDS FOR PETS WITH FENBENDAZOLE

  • PANACUR C for DOGS and CATS - ORAL GRANULES against ROUNDWORMS, HOOKWORMS, WHIPWORMS, LUNGWORMS and TAPEWORMS - MERCK ANIMAL HEALTH
  • SAFE-GUARD for DOGS - ORAL GRANULES against ROUNDWORMS, HOOKWORMS, WHIPWORMS and TAPEWORMS - MERCK ANIMAL HEALTH

PARASITE RESISTANCE

In livestock & horses: Yes, as all benzimidazoles, very frequent worldwide in gastrointestinal roundworms in sheep, goats, cattle & horses.
In dogs and cats: No

Learn more about parasite resistance and how it develops.


SPECIFIC FEATURES

Fenbendazole is a veteran anthelmintic (wormer) compound belonging to the chemical class of the benzimidazoles. Fenbendazole is one of generic anthelmintics most used on livestock worldwide. There are hundreds of brands.

For livestock it is available in the form of drenches, feed additives, bolus, tablets, pills, etc and slow-release devices, all for oral administration. There are no classic injectables or pour-ons with fenbendazole, excepting some drenches that can are approved for intraruminal injection in some countries.

For horses it is mostly available in feed additives and oral pastes.

For dogs and pets it is usually available in the form of drenches and tablets, pills, etc.

Both for livestock and pets it is very often used in combinations that broaden the spectrum of activity. Typical mixtures for livestock include a flukicide (e.g. triclabendazole, closantel, etc.) and/or a macrocyclic lactone (typically ivermectin) and/or levamisole. For dogs and cats it is mostly combined with a taenicide (e.g. praziquantel).

Efficacy of fenbendazole

Fenbendazole has a broad-spectrum of activity against gastrointestinal roundworms and lungworms of livestock, including adults and L4-larvae of the most important species (e.g. of the genus Bunostomum, Haemonchus, Ostertagia - Teladorsagia, Trichostrongylus, Cooperia, Nematodirus, Chabertia, Oesophagostomum, Trichuris, Dictyocaulus, Muellerius, etc.) as well as arrested larvae of several species. It is also effective against most livestock tapeworms (e.g. Moniezia, Taenia).

In horses it controls the major parasitic roundworms such as Large Strongyles (Cyathostomins), Small Strongyles (Strongylus spp), Parascaris equorum, etc. as well as tapeworms. (e.g. Anaplocephala spp).

It is also effective against the major parasitic roundworms (e.g. Ancylostoma, Toxocara, Trichuris, Uncinaria) and tapeworms (e.g. Echinococcus, Dipylidium, Taenia, etc.) of dogs and cats.

Fenbendazole is absorbed slowly in the stomach. Therefore the longer it remains there, the better the efficacy. In carnivores (e.g. dogs and cats) and other animals with a simple stomach the passage through the stomach is rather fast and therefore a higher dosage is required. This is why it is more often used on livestock than on dogs and cats.

In ruminants, unless delivered using a slow-release device, fenbendazole has only a limited residual effect. This means that a single administration will kill the parasites present in the host at the time of treatment and protect against re-infestations for a few more days, but not for weeks or months. In non-ruminants the residual effect is substantially shorter, i.e. only a few hours.

At the therapeutic dose fenbendazole is not effective against flukes and whatsoever external parasites.

Unfortunately, resistance of several gastrointestinal roundworms to all benzimidazoles,, including fenbendazole is already very high and very frequent worldwide in sheep and goats, slightly lower in cattle. For this reason, the risk that benzimidazoles fail to protect ruminants against gastrointestinal roundworms is considerable worldwide. Resistance of worms to benzimidazoles in dogs, cats, pig and poultry are so far not a serious problem.

Pharmacokinetics of fenbendazole

Fenbendazole is very poorly soluble in water. Orally administered fenbendazole is poorly absorbed into the bloodstream. Therefore it is very important that it remains as long as possible in the rumen, to form a reservoir from which it is progressively solved and absorbed. Direct administration into the abomasum (e.g. due to the "oesophageal groove reflex") strongly diminishes the absorption and consequently its efficacy.

Absorbed fenbendazole is metabolized in the liver to its sulfoxide derivative, which is identical with oxfendazole, another veterinary anthelmintic benzimidazole. Interestingly, the oxfendazole produced through metabolism is released back to the rumen, where the bacterial flora reduces it back to fenbendazole. This increases the bioavailability of fenbendazole in ruminants.

Excretion occurs mainly through feces. In cattle and sheep, 6 days after oral administration about 35% of the ingested dose is excreted unchanged, and about 5% metabolized. A small amount is excreted through urine, mainly in the form of various metabolites. Excretion in goats is about twice as fast as in sheep.

In dogs, cats and birds, the absence of such a reservoir as in ruminants strongly shortens the residual effect, which may require a higher dose or more frequent treatments to achieve the desired efficacy. In dogs, 48 hours after administration fenbendazole is not detectable in blood plasma, in birds 36 hours, and in cats 7 days.

Influence of diet. In ruminants, reducing the amount of feed slows down the exit flow of the rumen and prolongs the time the anthelmintic remains there and can be absorbed. Consequently it is advisable to reduce the animals' access to feed (especially to fresh pasture, not to water) 24 hours before administration. For the same reason, it is better to keep the animals away from food for about 6 hours after drenching. However sick, weak, or pregnant animals should not be kept away from food and fasting animals should have access to water. In cattle, a fiber-rich diet also increases the bioavailability of fenbendazole.

In contrast with this, in dogs and horses, administration of fenbendazole with the food increases the bioavailability of fenbendazole.

Influence of parasites. Heavy infestations with gastrointestinal roundworms reduce the bioavailability of fenbendazole in ruminants. The reason is that the inflamed wall does not properly regulate the pH (acidity) in the abomasum and the intestine, which has a negative influence on the solubility and the absorption of fenbendazole and on the distribution of its metabolites. In addition, the passage of food through the stomach is also faster in case of heavy infestations, which reduces the bioavailability of the anthelmintic.

Mechanism of action of fenbendazole

The molecular mode of action of all benzimidazoles, including fenbendazole, consists in binding to tubulin, a structural protein of microtubules. These microtubules are important organelles involved in the motility, the division and the secretion processes of cells in all living organisms. In the worms the blocking of microtubules perturbs the uptake of glucose, which eventually empties the glycogen reserves. This blocks the whole energy management mechanism of the worms that are paralyzed and die or are expelled.

Since cell division is also disturbed, worm egg production and development is also blocked by benzimidazoles, i.e. most of them also have an ovicidal effect.

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